GARTH BROOKS is overjoyed at becoming a first-time grandpa –but he’s also brokenhearted that his daughter is an unwed teenage mom, say insiders.

The country kingpin’s daughter August gave birth on July 23 just two months after her 19th birthday, and Garth was thrilled that his new granddaughter arrived healthy and happy.

But the deeply spiritual, 51-year-old super­star is singing the blues that little Karalynn was born out of wedlock, and he may even delay his much-anticipated comeback to keep a close eye on all three of his daughters.

“Garth lightened up when he saw the baby,” revealed a source close to the family. “But he’d been upset with August for getting pregnant at such a young age when he had big plans for her.”

In stark contrast to the country star’s bitter­sweet feelings, August’s mom Sandy Brooks has been in seventh heaven over the arrival.

Sandy, Garth’s first wife and moth­er of his three daughters, has been locked in a valiant battle with breast cancer since 2006 – and despite the divorce Garth has been there every step of the way, say insiders.

“Sandy has been fighting bravely, but she knows she might not have much time left,” confided a friend. “Being able to cradle her grand­daughter in her arms is heaven for her.

“She’s so grateful that she’s lived long enough to see the bright eyes and lovely face of her grandchild.”

The baby’s mom is the middle child of the all-girl Brooks family that includes daughters Taylor, 21, and Allie, 17. She enrolled at the University of Oklahoma last fall and became pregnant by her high school sweetheart Chance Russell during her first semester, according to the close source.

The two live together and – despite Garth’s estimated $365 million fortune – have registered for baby gifts at Walmart and other stores, asking for a $300 baby crib, baby furni­ture, a stroller and diapers.

Meanwhile, university graduate Garth – who retired from his record-shattering career in 2000 to become a hands-on dad – expected all his kids to graduate college and now he’s unsure about August’s future, say sources.

But Sandy worries that he may be meddling too much in the girls’ lives.

“She told him he can’t be their Father Protector their whole lives,” disclosed the friend.

Garth, who married country star Trisha Yearwood in 2005, has been edging back into the music scene and even did a long stint in Las Vegas. But his decision whether to record and tour again certainly won’t be before his youngest daugh­ter graduates from high school.

“He’s come to see his new grand­daughter as an unexpected blessing, but it makes him more doubtful about firing up his career again,” said the source. “He feels he still has a lot of parenting to do.”